The present invention relates to barium titanate based compositions and, more particularly, relates to dispersible, submicron, doped barium titanate coforms with narrow particle size distributions.
Barium titanate based compositions are extensively used in the electronics industry for the production of capacitors, condensers and PTCR (positive temperature coefficient of resistance) devices. Barium titanate is particularly useful and versatile in electronic applications since its electrical properties can be substantially modified by the incorporation of additives and/or dopants. The additives which are frequently employed are MAO.sub.3 compounds, where M is a divalent cation and A is a tetravalent cation, having the BaTiO.sub.3 perovskite structure. Typical additives include the titanates, zirconates and stannates of calcium, strontium, barium and lead. Since the additive or additives have the same crystal structure as BaTiO.sub.3, they readily form a solid solution during calcination or sintering. In general, additives represent more than 3 mole % of the BaTiO.sub.3 based formulation. Dopants cover a wide range of metal oxides. These, in general, represent less than 5 mole % of the total BaTiO.sub.3 based formulation. The dopant or dopants employed may be completely or partially miscible in the perovskite lattice or may be immiscible in the lattice. Examples of dopants employed include the oxides of La, the lanthanides, Y, Nb, Ta, Cu, Mo W, Mn, Fe, Co, Ni, Zn, Al, Si, Sb and Bi. Small amounts of some of the dopants, for example, as discussed later, Sb, exhibit a substantial effect on the electrical properties of BaTiO.sub.3, based compositions.
In commercial practice, barium titanate based formulations are produced either by blending the required pure titanates, zirconates, stannates and dopants or by directly producing the desired powder by a high temperature solid state reaction of an intimate mixture of the appropriate stoichiometric amounts of the oxide or oxide precursors (e.g., carbonates, hydroxides or nitrates) of barium, calcium, titanium, etc. The pure titanates, zirconates, stannates, etc. are also, typically, produced by a high temperature solid phase reaction process.
The prior art processes for producing barium titanate and barium titanate based compositions by solid phase reactions are relatively simple; nevertheless, they do suffer from several disadvantages. Firstly, the milling steps serve as a source of contaminants which can adversely affect electrical properties. Secondly, compositional inhomogenieties, resulting from incomplete mixing on a microscale, can lead to the formation of undesirable phases such as barium orthotitanate, Ba.sub.2 TiO.sub.4, which can give rise to moisture sensitive properties. Thirdly, during calcination, substantial particle growth and interparticle sintering occur. As a consequence, the milled products consist of irregularly shaped fractured aggregates which have a wide size distribution ranging from about 0.2 to about 10 microns. It has been established that green bodies formed from such aggregated powders with broad aggregate size distributions require elevated sintering temperatures and give sintered bodies with broad grain size distributions. Finally, since commercial BaTiO.sub.3 based compositions can contain small but variable amounts of various impurities, each lot of BaTiO.sub.3 or BaTiO.sub.3 based composition produced must be qualified. The qualification procedure involves determination and, if required, modification of the electrical properties of the final sintered body by changing the levels of dopants employed.
Many approaches have been developed to try to overcome the limitations of the conventional solid state reaction processes. Precipitation of either doped barium titanyl oxalate or doped barium titanyl oxalate with partial substitution of strontium or lead for barium and zirconium for titanium is taught by Gallagher et al., "Preparation of Semiconducting Titanates by Chemical Methods," 46, J. Amer. Chem. Soc., 359 (1963); Schrey, "Effect of pH on the Chemical Preparation of Barium-Strontium Titanate," 48, J. Amer. Cer. Soc., 401 (1965) and Vincenzini, "Chemical Preparation of Doped BaTiO.sub.3," Proceedings of the Twelfth Intl. Conf., Science of Ceramics, Vol. 12, p. 151 (1983). The oxalates are decomposed at elevated temperature to form the doped barium titanate based compositions. U.S. Pat. No. 3,637,531 teaches heating a single solution of dopant, titanium compound and alkaline earth salts to form a semi-solid mass that is converted to the desired titanate based product by calcination. U.S. Pat. No. 4,537,865 discloses combining hydrous oxide precipitates of Ti, Zr, Sn, or Pb and hydrous oxides of the dopants with aqueous slurries of precipitated carbonates of Ba, Sr, Ca or Mg. The solids are calcined to give the required product. Kakegawa et al., "Synthesis of Nb-doped Barium Titanate Semiconductor by a Wet-Dry Combination Technique," 4, J. Mat. Sci. Lets., 1266 (1985) describe a similar synthesis procedure.
Mulder, "Preparation of BaTiO.sub.3 and Other Ceramic Powders by Coprecipitation of Citrates in an Alcohol", 49, Ceramic Bulletin, 990-993 (1970), prepares doped BaTiO.sub.3 and BaTiO.sub.3 based products by spraying an aqueous solution of citrates or formates of the constituents into an alcohol to effect dehydration and coprecipitation. The products obtained by calcination of the coprecipitated citrate or formate powders consist mostly of compact globules having sizes in the 3 to 10 micron range. U.S. Pat. No. 4,061,583 describes doped BaTiO.sub.3 based compositions prepared by addition of a solution of either the nitrates or chlorides of the required constituents to an aqueous alkaline solution containing hydrogen peroxide. Decomposition of the peroxide containing precipitate at about 100.degree. C. results in the formation of an amorphous BaTiO.sub.3 based composition. Calcination of the amorphous product to about 600.degree. C. gives crystalline powders. Unfortunately, the primary particle sizes of the products are not characterized. Replication of some of the examples given in the patent indicated that the amorphous powders had primary particle sizes which were substantially smaller than 0.05 microns. Transmission electron micrographs of the products showed that the primary particles of the 600.degree. C. calcined products were aggregated.
In the above examples of typical prior art processes, calcination is employed to complete the synthesis of the particles of the desired compositions. For reasons already noted, this elevated temperature operation is deleterious as it produces aggregated products which after comminution give smaller aggregate fragments with wide size distributions.
U.S. Pat. Nos. 4,233,282; 4,293,534 and 4,487,755 describe synthesizing BaTiO.sub.3 and BaTiO.sub.3 based compositions through a molten salt reaction in which Ba is partially replaced by Sr and Ti is partially replaced by Zr. The products are characterized as being chemically homogeneous and consisting of relatively monodisperse submicron crystallites. Doped BaTiO.sub.3 based products were not synthesized. Yoon et al., "Influence of the PTCR Effect in Semiconductive BaTiO.sub.3," 21, Mat. Res. Bul., 1429 (1986), teaches employing a molten salt process to synthesize products having the composition Ba.sub.(0.900-x) Sr.sub.0.1OO Sb.sub.x TiO.sub.3 where x has the values 0.001, 0.002, 0.003 and 0.004. The bodies produced from the molten salt process exhibited greater effects on the PTCR in their resistivity-temperature characteristics and large resistivities at room temperature and larger current variations in current-time characteristics than the comparable specimens formed from powders produced by calcination of a mixture of the oxides and oxide precursors. The differences were attributed to the use of KCl in the molten salt synthesis process and to the smaller size and size distribution of the grains in the samples derived. Although the molten salt based synthesis process can be used to give submicron doped products with narrow size distribution, the powders are inevitably contaminated with alkali metals, since the molten salts consist of alkali metal salts. Of course, in most electronic applications alkali metals are deleterious contaminants.
Several aqueous based processes have been described for producing BaTiO.sub.3 as well as BaTiO.sub.3 based compositions where Ba is partially replaced by Sr and Ti is partially replaced by Sn or, possibly, by Zr. In the process taught in U.S. Pat. No. 3,577,487, doped multicomponent alkaline earth and/or Pb(II) titanates, stannates, zirconates and/or hafnates are prepared. In these cases either the coprecipitated hydrogels are treated with alkaline earth hydroxides and subjected to the same treatment steps as those used for producing BaTiO.sub.3 or the required gels and alkaline earth hydroxides are added to a preformed BaTiO.sub.3 slurry which is then subjected to fluid energy milling and calcination. Unfortunately, the products prior to fluid energy milling, were not characterized. However, experience would indicate that the doped multicomponent products, prior to milling, should have specific surface areas in excess of 20 m.sup.2 /g which indicates that the powder primary particle sizes are less than about 0.05 microns. Even after fluid energy milling at outlet temperatures in excess of 800.degree. F., the multicomponent products cited in the examples had specific surface areas in excess of 18 m.sup.2 /g. Calcination results in a further decrease in specific surface area. This, for reasons already discussed, will lead to the formation of aggregated products.
A publication of the Sakai Chemical Industry Company entitled "Easily Sinterable BaTiO.sup.3 Powder" by Abe et al. discloses a hydrothermal process for synthesizing barium titanate based coforms with the formula BaTi.sub.(1-x) Sn.sub.x O.sub.3. It is expected that the morphologies of the Sn-containing coforms are comparable with those of this invention. However, Abe et al. is limited in that it teaches only the synthesis of Sn-containing BaTiO.sub.3 based compositions. Perhaps, by analogy, it does suggest the use of other tetravalent cations such as Zr(IV) and, possibly, the use of Sr(II), since, like Ba(OH).sub.2, Sr(OH).sub.2 is quite soluble in aqueous media.
In our U.S. patent application Ser. No. 859,577, multicomponent powders having the general formula EQU Ba.sub.(1-x-x'-x") M.sub.x M'.sub.x' M".sub.x" Ti.sub.(1-y-y'-y") A.sub.y A'.sub.y' A".sub.y" O.sub.3
are disclosed where M equals Pb(II), M' equals Ca(II), M" equals Sr(II), A equals Sn(IV), A' equals Zr(IV) and A" equals Hf(IV), x, x', x" and y, y', and y" represent the atom fractions of the divalent and tetravalent cations, respectively, x.sup.D ", y, y' and y" each having independent values ranging from 0 to 0.3 and x" and x' each have independent values ranging from 0.01 to 0.3, so long as the sum of either (x+x'+x") or (y+y'+y") does not exceed 0.4. The products having the above nominal stoichiometries were produced in a general hydrothermal process and were termed coforms. Each of the coforms was characterized as being stoichiometric, dispersible, submicron and having a narrow particle size distribution.
Doping of the barium titanate coforms was not investigated either in U.S. application Ser. No. 859,577 or in Abe et al. Hence, there is absent in the prior art any doped coforms of barium titanate which include calcium and/or lead or multiple divalent and tetravalent cation substitutions which are dispersible, spherical and submicron with narrow particle size distributions except when these reagents are present at impurity levels. For example, Abe et al. found that the maximum level of any impurity in their hydrothermally derived BaTiO.sub.3 product was 0.01 weight %. In practice, it may be expected that the amounts of impurities present in precipitated BaTiO.sub.3 and BaTiO.sub.3 based compositions will vary with the source of the reactants employed. From an examination of the purities of a number of commercially available reactants or reactant precursors, such as TiCl.sub.4, ZrO(NO.sub.3).sub.2, Ba(OH).sub.2, Sr(OH).sub.2 PbO, Ca(OH).sub.2, CaCO.sub.3, and SnCl.sub.4, it is concluded that the level of a dopant impurity to be found in prior art precipitated BaTiO.sub.3 and BaTiO.sub.3 based compositions, including those described in the copending 859,577 application will be much smaller than 0.1 weight %. In other words the atom fraction of an impurity such as aluminum, having an atomic weight of 27, present will be less than 0.009. Most dopants have much larger atomic weights and, even if present at the high level of 0.1 weight %, would have atom fractions which are smaller than 0.009.
As noted earlier certain dopants, even when present at impurity levels, affect the electrical properties of BaTiO.sub.3. Nevertheless, the atom fractions of dopants present in practical BaTiO.sub.3 based dielectric compositions, typically, have values which exceed 0.009 and, more preferably, exceed 0.01.
Accordingly, it is a primary object of the present invention to provide a dispersible, submicron doped barium titanate coform with a narrow particle size distribution.
It is another object of the present invention to provide a wide variety of doped BaTiO.sub.3 based compositions having primary particle sizes in the size range between 0.05 and 0.4 microns.
It is another object of the present invention to provide a doped barium titanate based composition having exquiaxed primary particles.
It is another object of the present invention to provide doped barium titanate based compositions that are substantially free of mill media.
It is a still further object to provide doped barium titanate based compositions in which all constituents are intimately mixed on a particle size scale.